The main components of paper are cellulose and hemicellulose. The plant fibers for papermaking include wood fiber and non-wood fiber. The main compositions of plant fibers are cellulose, hemicellulose, and lignin. Paper pulping is a process of removing lignin as much as possible and retaining cellulose and hemicellulose. Wood fiber is a high-quality papermaking material and it has a great advantage in the paper industry. With the rapid development of industry, the contradiction between supply and demand of wood, the search for non-wood fiber paper raw material is a realistic and urgent problem to be solved.

Rice straw is a cheap non-wood fiber material for papermaking. Since the fiber length ratio is large, which is good for interweaving and bonding between fibers, and increase the smoothness and transparency of the paper. Bleached straw pulp can be applied into a common culture paper making, also can be used with wood pulp to make high-quality printing paper and writing paper.

Rice straw pulping process

Rice straw is an annual herbaceous stem plant with high hemicellulose content, low lignin content, short and fine fibers, high content of non-fibrous cells, especially thin wall cell content is high, so the water filtration performance of the straw pulp is poor, and the ash content of pulp is high. The straw section is solid and difficult to cook. Therefore, the study of straw cooking process will help to further optimize the straw cooking process.

As we all know, rice straw pulp making usually adopts displacement cooking, so the pulping equipment includes displacement digester system, blow tank, vacuum drum washer, twin roll press, single screw press, and etc.

According to the data, the content of SiO₂ of straw is much higher than that of other fiber materials. For common soda cooking: Generally, the SiO₂ content of various fibers is different, the wood pulp black liquor solids are less than 1%; the sputum and bagasse black liquor is 5% to 8%, and the straw black liquor is as high as about 20%.

components of rice straw

 

After the dry and wet material preparation, the grass pieces are transported through the screw conveyor of the wet preparation section to straw metering section of the continuous cooking section. After being metered and then enter into the heating screw conveyor, and the pre-heat treated grass pieces are subsequently fed into the thickening feeder. The meter controls the amount of feed by the rotation speed, and the heating screw conveyor is introduced into the low-pressure steam, which is beneficial to the softening of the grass piece and the immersion of the liquid medicine, thereby achieving the purpose of pre-steaming.

Notes for rice straw pulp making

1. The fiber dissociation point is the pulp yield or pulp hardness during the disintegration of the plant fiber in the cooking process and is an ideal endpoint for chemical pulping. When the fiber material is cooked to reach the fiber dissociation point, the lignin in the intercellular layer of the fiber material is substantially removed, and the residual lignin of pulp is mainly the lignin in the fiber cell wall, and if the cooking is continued, The further removal of lignin in the cell wall and the degradation of carbohydrates are also exacerbated. The fiber dissociation point can usually be determined according to the curve of the slag rate in the pulp after cooking.
2. The removal of lignin from straw fiber materials in the alkaline cooking process is mainly divided into three stages: a large amount of delignification stage, a supplementary delignification stage, and a residual delignification stage. It is mainly divided according to the curve of the lignin content in the crude pulp at different stages of cooking. The lignin in the black liquor can approximate the lignin dissolved in the pulp. Therefore, the content of acid-insoluble lignin and acid-soluble lignin in the black liquor is determined, and the sum of the two is the total lignin dissolved in the pulp.
3. In the cost structure of pulp, chemical products are the main cost in addition to plant fiber materials. Therefore, the study of the consumption of chemicals helps to improve the cooking process and reduce the number of chemicals.
4. The purpose of cooking is the removal of lignin, dissociate the fibers, and minimize the degradation of cellulose and hemicellulose. Therefore, understanding the dissolution of carbohydrates at different stages of the cooking process during straw pulping helps to adjust and control the cooking process, reduce carbohydrate damage, and increase pulp yield.